Method for heating a blast furnace stove

ABSTRACT

A method for heating a blast furnace stove by combusting in a stable, visible flame a fuel with a lower heating value (LHV) of 9 MJ/Nm 3  or less in a combustion region, arranged in a combustion chamber in the stove, and causing the combustion gases to flow through and thereby heat refractory material in the stove, wherein the fuel is combusted with an oxidant comprising at least 85% oxygen, and combustion gases are caused to be recirculated into the combustion zone and thereby dilute the mixture of fuel and oxidant therein sufficiently for the flame not to damage the refactory material.

BACKGROUND OF THE INVENTION

The present invention relates to a method for heating a blast furnacestove for use with a blast furnace.

The combustion air supplied to a blast furnace is typically preheatedusing a stove, comprising refractory material which is heated using aburner. When the material is hot enough, combustion air is passedthrough the stoves to pre-heat it before injection into the blastfurnace. Usually, several stoves are operated in parallel and cyclicallyso that at least one stove is operated for heating combustion air whilethe refractory material of at least one stove is heated.

Conventionally, the top gas leaving the blast furnace has a temperatureof around 110-120° C. and contains about 20-25% each of CO and CO₂.Typically, 3-5% H₂ and some H₂O will also be present, but the othermajor constituent of the top gas is N₂ (typically 45-57%). The gasconstitutes a low grade fuel, having a relatively low heating value, andis commonly used to fuel the stoves.

The top gas is normally combusted using air-fuel burners in the stoves.In order to ensure the necessary high air blast temperatures needed bythe blast furnace, it is well known to enrich the top gas with a highcalorific value gas, such as coke oven gas or natural gas. Thecombustion of such additional fuel leads to larger overall emissions ofcarbon dioxide from the plant, and is therefore not desirable.

It is also known to oxygen enrich the combustion air used in stackburners. Usually, the enrichment levels needed to reduce or eliminatethe need for additional, high-calorific fuels are such as to result in afinal oxidant oxygen content in the combustion air of around 28-30%.

Such methods may in some cases render peak flame temperatures highenough to damage the refractory material of the stove.

The blast furnace itself is a highly efficient counter-current reactorthat has evolved over many years. It is approaching the limits ofthermodynamic efficiency. Moreover, the blast furnace and its ancillaryequipment, such as stoves, are the largest energy consumers in anintegrated iron and steel works. Furthermore, the energy consumed iniron making is the dominant factor determining the carbon consumption ofthe integrated steel making process, and therefore the emissions ofcarbon dioxide. Therefore, it would be desirable to increase thermalefficiency of blast furnace stoves.

In addition to the problem of high peak temperatures mentioned above,too low flame temperatures or heat input rates will lead to long heatingcycles, which is undesirable. In other words, the flame temperatureneeds to be moderated.

SUMMARY OF THE INVENTION

The present embodiments solve the above described problems and make itpossible to achieve other advantages as will be described below.

Thus, the present embodiments relate to a method for heating a blastfurnace stove by combusting a fuel with a lower heating value (LHV) of 9MJ/Nm3 or less in a combustion region in which there is maintained astable visible flame, arranged in a combustion chamber in the stove, andcausing the combustion gases to flow through and thereby heat refractorymaterial in the stove, and is characterized in that the fuel iscombusted with an oxidant comprising at least 85% oxygen, and in thatcombustion gases are caused to be recirculated into the combustionregion to dilute the mixture of fuel and oxidant therein sufficientlyfor the flame not to damage the refactory material.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in detail, withreference to exemplifying embodiments of the invention and to theappended drawings, in which:

FIG. 1 is a simplified illustration of a blast furnace and three stovesin a conventional iron works;

FIG. 2 is a section view illustrating a conventional stove of a moderntype with external combustion chamber;

FIG. 3 is a section view of a stove with additional lances according tothe present embodiments;

FIG. 4 is a detail section view of a stove with an oxyfuel burneraccording to the present embodiments;

FIG. 5 is a section view of a stove with combustion gas recyclingaccording to the present embodiments;

FIG. 6 is a detail section view of a stove with an ejector lanceaccording to the present embodiments;

FIG. 7 is a graph illustrating the axial temperature profile forcombustion in the combustion chamber of a burner stove (a) operatedconventionally with air supporting combustion and without recirculationof the flue gas and (b) operated in accordance with the embodiments;

FIG. 8 is a graph similar to FIG. 7, but showing the axial velocityprofile for the same two combustion cases; and

FIG. 9 is a graph similar to FIG. 7 but showing the axial carbonmonoxide concentration profile for the same two combustion cases.

DETAILED DESCRIPTION

FIG. 1 illustrates the principal arrangement of a blast furnace 120 andthree stoves 100 in an iron works. The operation of the blast furnace120 produces blast furnace top gas, which is fed, using a fuel supplycontrol device 110, to each stove 100 to be used as fuel to heat thestove 100 in question. The top gas is combusted with an oxidant in theform of air, which is supplied by an air supply control device 130.

Each stove 100 comprises refractory material in the form of ceramicbricks or the like, which is first heated and then used to heat blastair which is fed into the blast furnace.

When operated in refractory material heating mode (“on gas” mode), thetop gas is combusted in the stove 100 with the oxidant, and thecombustion gases are fed to a flue gas treatment device 150, possiblyincluding a conventional carbon capture step.

When operated in blast air heating mode (“on blast” mode), air is ledthrough the refractory material in the opposite direction, and then onto the blast furnace 120.

The stoves 100 are operated cyclically, so that at any point in time atleast one stove is operated on blast and the rest of the stoves areoperated on gas.

FIG. 2 is a section view through a conventional stove 100 of a moderntype. The stove 100 comprises an external combustion chamber 101,refractory material 102 and a dome 103. When operated on gas, it iscritical that the temperature in the dome 103 does not become too high,since there is then a risk of damage to the stove 100. It is to beunderstood that there are also stoves with internal combustion chambers,and that the present invention is equally applicable to the operation ofsuch stoves.

When operated on gas, top gas and air is fed into a combustion zone ofthe combustion chamber 101, in which combustion takes place, via an airburner 108. The burner 108 comprises a fuel inlet 105 and an air inlet104. The hot combustion gases then stream up through the chamber 101,past the dome 103 and down through the refractory material 102, therebyheating the latter. When exiting through the port 106, the temperatureof the combustion gases is conventionally about 200-350° C.

When the refractory material has reached a predetermined temperature,the operation is switched to on blast operation. Then, air is introducedthrough the port 106, streams through the hot refractory material 102,via the dome 103 and the combustion chamber 101, and out through anoutlet port 107. At this point, the blast air has a typical temperatureof 1100-1200° C.

It is preferred, in the context of the present invention, to heat thestove with blast furnace top gas, as described above. It is furthermorepreferred to use top gas from a blast furnace to which blast air isprovided from the stove. This allows for the arrangement of the stovenear the blast furnace, is energy efficient and leads to low totalemissions from the plant.

However, it is to be understood that the present invention can beequally advantageously applied to stoves heated with other low-gradefuels. By way of example, typical chemical compositions (percentagevalues) and lower heating values (LHV) are provided in Tables I and II,respectively, for blast furnace top gas and converter off-gas.

TABLE 1 N₂ O₂ H₂ CO CO₂ CH₄ C_(m)H_(n) H₂O Top gas 52.5 0.55 2.3 23.5 20— — 1.15 Off-gas 17.2 0.1 2.5 64.5 15.6 — — 0.1

TABLE 2 LHV (MJ/Nm³) LHV (MJ/kg) Top gas 3.2 2.4 Off-gas 6.3 8.4

According to the present invention, the stove is heated with a gaseousfuel the LHV value of which is not higher than 9 MJ/Nm³. Use of suchlow-grade fuel will draw maximum benefit from the possible cost benefitsof the present invention. The fuel may comprise a certain addition ofanother, more high-grade fuel, as long as the LHV value of the mixtureis equal to or less than 9 MJ/Nm³. In order to minimize cost andemissions, it is however preferred not to add high grade fuels prior tocombustion.

According to the present invention, such a low-grade fuel is used forheating the stove by combusting it, not with air or slightlyoxygen-enriched air, but with an oxidant comprising at least 85% byweight, preferably at least 95% by weight, oxygen, where the oxidantmost preferably is industrially pure oxygen having an oxygen content ofessentially 100%.

This will increase fuel efficiency, since the nitrogen ballast presentin air does not need to be heated. Moreover, by reducing the nitrogenballast in the combustion products, the necessary flame temperatures canbe attained without the need to supplement the low-grade fuel gas withhigh calorific fuels. The reduced energy demand will facilitateincreased power generation and/or lead to a reduced need for import gas,thus improving fuel management.

Normally, using an oxidant with such large oxygen contents would lead topeak temperatures high enough to damage the dome and refractory materialof the stove.

However, it is possible to use this type of oxidant under condition thatthe stove combustion gases are recirculated into the combustion zone tosuch extent that the mixture of fuel and oxidant therein is dilutedsufficiently for the combustion in the combustion region to form astable, visible flame at temperatures that do not damage the dome andthe refractory material.

That “combustion gases are recirculated into the combustion region”herein refers to combustion gases located outside of the combustionregion being recirculated back into the combustion region. Suchcombustion gases may originally be located inside the combustion chamberitself, but outside of the part of the combustion chamber occupied bythe region in which combustion mainly takes place (the “combustionregion”). Thus, in this case combustion gases are in fact recirculatedwithin the combustion chamber. Alternatively, such combustion gases maybe recirculated from outside of the combustion chamber back to thecombustion region.

As will be described in further detail in the following, the dilution ofthe reactants may be achieved either by creating heavy turbulence insidethe combustion chamber using high-velocity lancing of oxidant, possiblyusing a staged combustion scheme, and/or the recycling of flue gasesfrom the stove back into the combustion zone.

In accordance with the invention, it is possible to achieve sufficientlylow peak flame temperatures so as not to damage the refactory materialof the stove.

Additionally, when a high-oxygen oxidant is used to combust low-gradefuels such as blast furnace top gas, the CO₂ contents of the combustiongases become considerably higher as compared to when using air orslightly oxygen-enriched air as the oxidant. Since conventional carboncapture techniques tend to be considerably cheaper per unit captured CO₂when the treated gas contains a larger share of carbon dioxide, thisleads to considerable cost savings when using such a carbon capture stepto treat the stove combustion gases.

FIG. 3 shows a preferred embodiment of the invention. A stove 300, whichis similar to the conventional one 200 shown in FIG. 2, comprises acombustion chamber 301, refractory material 302, a dome 303, an inlet304 used for combustion air when the stove is operated in a conventionalmanner with air combustion, another inlet 305 used for low-grade fuelsuch as top gas, and ports 306, 307 similar to ports 206, 207. Insteadof combusting the low-grade fuel with air, one or several lances 310,311, 312 are inserted into the combustion chamber, and are used tosupply the above defined high-oxygen oxidant into the combustion zone.The oxidant may be provided by local oxygen production or using anexternally provided oxidant.

In all embodiments described herein, the total amount of oxidant pertime unit is balanced against the amount of supplied low-grade fuel, soas to create the desired combustion conditions in terms ofstoichiometry.

It is preferred that each lance 310, 311, 312 supplies oxidant to thecombustion zone at high velocity, preferably at least 200 m/s, morepreferably at least sonic velocity. Such high-velocity lancing leads toheavy turbulence in the combustion chamber, in turn entrainingcombustion gases into the combustion zone and thereby diluting the flameso as to render it diffuse with a peak temperature that does not damagethe refractory material of the stove.

According to one preferred embodiment, a lance 310 is arranged with itsorifice in close proximity to the orifice of the fuel inlet 305.According to another preferred embodiment, a lance 311 is arranged at aposition at a distance from the orifice of the fuel inlet 305. Dependingon the geometry of the combustion chamber 301, one of thesearrangements, or a combination of both, may provide the bestrecirculation of combustion gases into the combustion zone. Asupplementary lance 312, arranged further downstream in relation to theother lance or lance 310, 311, can be used to provide a stagedcombustion process, whereby the total flame volume can be made evenlarger. Naturally, more than one lance of each of the described types310, 311, 312 may be arranged to complement each other. In case theoxidant is lanced in close proximity to the fuel inlet 305, it ispreferred to also lance oxidant further downstream so as to create astaged combustion process.

FIG. 4 is an overview illustration of another preferred embodiment, inwhich a blast furnace stove 400 comprises a combustion chamber 401,refractory material 402 and a port 406.

Low grade fuel is supplied via a supply conduit 411, a supply device 412and an inlet 413. Oxidant is supplied via a supply conduit 414, a supplydevice 415 and a lance comprising an orifice 416. The lance is arrangedso that its orifice 416 is arranged adjacent to the fuel inlet 413.Preferably, the lance runs coaxially to the fuel inlet 413, as depictedin FIG. 6. By such an adjacent arrangement, especially when coaxial, andwhen the oxidant is lanced at the above described high velocities, thefuel is efficiently entrained into the combustion zone by ejector actionon the part of the high velocity oxidant. As a result, heavyrecirculation of combustion products is achieved in the combustionchamber 401, in particular recirculating combustion gases into thecombustion zone expanding the flame front. When such a high-velocitylance is arranged adjacent to the fuel inlet 413, it is preferred tosimultaneously use a secondary oxidant lance 312, providing part of thetotally supplied oxygen at another location in the combustion chamber401 downstream of the fuel inlet 413, creating a staged combustion ofthe low-grade fuel and thereby facilitating the achievement of a flamewhich is diffuse and which does not have a peak temperature sufficientlyhigh to damage the refactory material of the stove.

The stove 400 can be part of a standing iron making plant and adapted tooperate in accordance with the invention from a conventional mode ofoperation in which air is used to support combustion of the blastfurnace gas, in which the blast furnace gas is supplemented by a cokeoven gas or natural gas and in which there is no recirculation of thecombustion products with the stove 400.

According to a preferred embodiment, an existing, conventional, airburner, which was used to heat the existing stove 400 previously, is inan initial step replaced by an oxyfuel burner 410 comprising the abovedescribed fuel inlet 413 and oxidant lance. An “oxyfuel” burner hereinrefers to a burner driven with a fuel and an oxidant, wherein theoxidant comprises a large part oxygen, preferably at least 85% oxygen,more preferably at least 95% oxygen.

According to an alternative, preferred embodiment, the existing airburner described above is, in an initial step, supplemented with one orseveral high-velocity oxidant lances as described above, and the airsupply is terminated.

As described above, such high velocity lancing yields heavy turbulenceinside the combustion chamber 301, 401, leading to sufficiently low peakflame temperatures for the refactory materials in the stove not to bedamaged.

However, the mass flow rate of the combustion gases will be lower whenusing a high-oxygen oxidant as compared to when using air as theoxidant. This would lead to smaller convective heat transfer to therefractory material and hence longer heating cycle times. Therefore,when converting an existing stove for high-oxygen oxidant operation,flue gases are recycled from the stove back into the combustion zone asdescribed below in connection to FIGS. 5 and 6.

Thus, FIG. 5 is an overview illustration of a stove 500 according toanother preferred embodiment, comprising a combustion chamber 501,refractory material (sometimes referred to as “checker work”) 502 and adome 503.

During on gas operation, the combustion gases leave the stove 500through a port 506. However, part of the combustion gases are recycledback to the combustion region in the combustion chamber 501 via arecycling device 511. The feedback device 511 may include a propellingdevice, such as a fan, to feed the recycled combustion gas to thecombustion chamber 501.

The recycling device 511 is also arranged to mix the recycled combustiongas with a high-oxygen oxidant of a composition as described above,provided via a supply conduit 512. The mixing may take place usingconventional diffusers. The mixture of recycled combustion gas andoxidant is then supplied to the combustion chamber 501 via an inlet 513.A low-grade fuel, such as top gas from the blast furnace, is provided,via a supply conduit 514, a supply device 515 and an inlet 516. In thecombustion zone, the fuel is hence combusted with the oxidant in thepresence of the combustion gases that have been recycled into thecombustion zone after they have already past the stove 500. This way,the flame in the combustion chamber 501 is diluted.

Using such flue gas recycling, it is possible to reach convective heattransfer rates high enough so as to be able to maintain the heatingcycle time of an existing stove in which a method according to thepresent invention is applied. This is achieved by recycling a sufficientamount of combustion gases to maintain the gas mass or thermal energyflow per time unit through the stove 500, at a level which is at leastthe same as the gas mass or thermal energy flow per time unit which wasused when the existing stove was operated, prior to conversion tooperation according to the present invention, using a low-oxygen oxidantwith no recycling.

As previously mentioned, the method according to the invention replacesair combustion of a calorifically enriched low calorific value fuel gaswith oxy-fuel combustion, in which the flame is diluted by recirculatingflue gas, by example by high impulse mixing of the combustion spaceusing lances for the injection of the oxidant. The need for a high costhigh calorific value booster fuel gas is eliminated and the stove isfuelled using blast furnace gas alone. The stoves typically account foraround 10% of the total energy demand for integrated steel-making andsome 18% of the energy delivered to the stoves is lost in the flue gas.Recycling flue gas reduces this energy loss and lowers the amount ofenergy that must be supplied to the stove from combustion of a fuel gas.The method according to the invention therefore combines some of thebenefits of waste heat recovery with those of oxy-fuel combustion.

Consider a hypothetical example of a 1500 m³ working volume blastfurnace operating with a productivity of about 2.2 t/m³/d. Such afurnace would produce some 138 tonnes of hot metal per hour and based ontypical blast volumes, might be expected to consume 138,000 Nm3/h of hotblast. To achieve a hot blast temperature of 1200° C. would require astove burner flame temperature about 150° C. higher and some 230 GJ/hwould be required to heat the air to this temperature. For a stoveefficiency of around 80% this means the energy input to the stoves wouldbe around 290 GJ/h or 145 GJ/h for each stove assuming two stoves are‘on-gas’ simultaneously. It is well established that for normal stoveoperating conditions, about 18% of the energy input to the stoves exitsin the flue gas. It has been estimated that for the conditionsconsidered this would result in a flue gas temperature of around 250° C.

These conditions have been used to establish hypothetical heat and massbalances for 3 modes of operation, “air-fuel” (i.e. conventionaloperation without flue gas recirculation; “oxygen-enriched” (i.e. as“air-fuel”, but with the air enriched in oxygen) and “flue gas recycle”(i.e. in accordance with the method of the invention). The calculationshave been done to ensure a constant flame temperature and constant massflow of combustion products, so that conditions for convective heattransfer are maintained. Flame stoichiometry has, in each case, beenadjusted to ensure 1% excess oxygen in the flue gas. The results arecompared in Table 3.

TABLE 3 Flue Flue Heat of Flame Mass Flue Flue BFG COG Air OxygenRecycle Combustion Temp. Flow Gas Gas Nm3/h Nm3/h Nm3/h Nm3/h Nm3/h GJ/h° C. kg/min % O₂ % CO₂ Air-Fuel 34000 2400 34200 0 0 145 1347 1539 1 23Oxygen 40200 1200 26800 1300 0 145 1347 1545 1 27 Enriched Flue Gas44700 0 0 6220 14490 139 1347 1541 1 41 Recycle

It can be seen that for the conditions considered, oxygen enrichment ofthe air supplied to the stoves reduces, but does not eliminate, theamount of coke oven gas employed. Blast furnace gas flow is increased toensure the heat input of 145 GJ/h is maintained. The CO₂ content of theflue gas increases marginally due to the elimination of some nitrogenfrom the system.

The introduction of flue gas recirculation removes the need forcalorific enrichment of the fuel gas. This is because a further modestincrease in the flow of blast furnace gas, combined with the recovery ofthe sensible heat contained in the flue gas, is sufficient to enable thedesired flame temperature to be reached. It is to be understood thatwith flue gas recirculation, the oxidant is not air but a gas mixturecontaining at least 85% by volume of oxygen or essentially pure oxygen.(The calculated results shown in Table 3 are based on the latter). Theenergy input from combustion is decreased by about 4% due to recovery ofenergy from the recycled flue gas.

Air is eliminated and combustion is sustained by the use of industrialoxygen. Importantly it can be seen that the CO₂ content of the flue gashas increased from the initial 23% to 41%. This equates to 50 tonnes ofCO₂ per hour for a single stove or 100 tonnes for the two stoves‘on-gas’. 75 tonnes of this would be available for carbon capture andsequestration whilst the remainder is recycled.

For the hypothetical case under consideration it is reasonable to assumethat the 138 tonnes of hot metal produced each hour is converted to 150tonnes of slab or other metal product which accounts for the likelihoodof scrap additions during steel-making.

Applying industry benchmark figures, it can be estimated that the entireintegrated steel plant would generate about 280 tonnes of CO₂ per hour.Hence, for the example considered, recycling flue gas to the stoves(assumed to be Cowper stoves) makes some 27% of the plant-wide CO₂emissions available for carbon capture.

Whilst simple heat and mass balances, such as those detailed in Table 1,serve to illustrate the main advantages achievable by the methodaccording to the invention, they do not completely reflect the benefits.In particular, they do not take account of the improved heat transferconditions generated on switching from air-fuel to oxy-fuel combustion.For this purpose a dynamic model that accounts for changes to theoverall heat transfer coefficient as a function of composition,temperature and mass flow in the refactory checker work can be used. Anumber of modelling studies of hot blast stoves, have shown that theheat transfer taking place can be accurately represented by an overallor ‘lumped’ heat transfer coefficient that combines the effects ofconvection and radiation. So that for the gassing cycle

α=α_(c)+α_(r)

Where;

α=convective heat transfer coefficient,and;α_(r)=radiative heat transfer coefficientThe convective heat transfer coefficient is related to the mass flowrate and may be calculated from the Sieder-Tate or Hausen equations. Theradiant heat transfer coefficient is derived from the Stefan-Boltzmannlaw which can be expressed in the form;

$h_{r} = {1.713 \times {10^{- 8}\left\lbrack \frac{{ɛ_{g} \cdot T_{g}^{4}} - {\alpha_{g} \cdot T_{B}^{4}}}{T_{g} - T_{B}} \right\rbrack}}$

Where;

ε_(g)=emissivity of the gas which is a function of composition andtemperature and may be derived from grey gas models or from Hottelcharts.α_(g)=absorptivity of the gasT_(g)=gas temperature.T_(B)=mean temperature of local checker work.

A zonal model that incorporates such principles and accounts for heattransfer to and within the checkers (refactory) has been used to make amore detailed assessment of the benefits. The baseline for comparison isoperational data from a set of modern Cowper stoves generating anindustry benchmark hot-blast temperature of 1250° C. The results areshown in Table 4.

TABLE 4 Conventional Oxygen Oxygen Oxygen Operations Case 1 Case2 Case 3Blast Cycle 30 30 25 25 Gas Cycle (8 52 52 42 42 minute change) BFGNm3/h 91,237 133,742 134,636 147,292 BFG HV MJ/Nm3 3.1 3.1 3.1 3.1Natural Gas Nm3/h 4,893 1,224 1,262 1,296 NG HV MJ/Nm3 33.9 33.9 33.933.9 Oxygen Rate per stove Nm3/h 0.00 23,665 21,854 23,665 Total HeatInput per stove MJ/h 448,707 456,094 460,153 500,540 per stove MJ/cycle388,879 395,281 322,107 350,378 total - 3 stove MJ/h 777,759 790,563773,058 840,907 operation Wind Rate Nm3/h 427,210 427,210 427,210427,210 Cold Blast ° C. 200 200 200 200 Hot Blast ° C. 1248 1246 12481261 Dome ° C. 1385 1385 1383 1384 Temperature Final Stack ° C. 399 399375 399 Temperature Projected Annual NA 1,554,375 2,660,237 4,611,263Cost Saving 

Stove Flue Gas Nm3/h 205,875 196,076 201,981 207,466 Volume CO₂ 23.05%45.43% 45.43% 45.43% Nm3/h - CO₂ 47,454 89,077 91,760 94,189

It is interesting to compare these cases in a little more detail:

The conventional operations show that the stove uses a significant levelof natural gas enrichment to generate a high blast temperature of 1248°C.

All three examples (“Oxygen Cases” 1, 2 and 3) are in accordance withthe inventors. In ‘Oxygen Case 1’ the model has been run retaining thesame blast temperature, blast volume, and stack temperature as in theconventional operations. This case generates comparable results to asteady state heat balance because although radiation heat transfer tothe (refractory) checker bricks has improved, the benefit of this isdisguised by forcing the model to retain a constant stack temperature.In fact, since the heat capacity of the CO₂ contained in the recycledflue gas, is higher than that of the nitrogen it is replacing, theoverall impact is that slightly more energy is needed to maintain aconstant dome (and blast) temperature. Nevertheless, replacing expensivenatural gas with a cheaper fuel source is sufficient to compensate forboth the higher energy input and the cost of the oxygen consumed. It isworth pointing out that the overall heat transfer coefficientscalculated by the model show a 13.5% increase relative to air fuelcombustion near the top of the checkers, but even at the lowertemperatures towards the base of the checkers the overall heat transfercoefficient had increased by some 8.5%.

In ‘Oxygen Case 2’ the enhanced heat transfer conditions have beenaccounted for by allowing the stack gas temperature to re-equilibrate toa lower temperature. It can be seen that since more heat is retained inthe checkers, the stack temperature drops by some 25° C. The net effectis that it is possible to lower gassing cycle time whilst retaining thesame blast temperature. The total energy input for a 3 stove operationis marginally reduced but blast temperature and volume is maintainedeven at a lower stack gas volume. This is an important feature thatcould be exploited under plugged stove conditions.

In ‘Oxygen Case 3’ the reduction in stack gas temperature by increasingthe firing rate until the original stack gas temperature has beenexploited by increasing the firing rate until the original stack gastemperature has been restored. It is apparent that the firing rate canbe increased by almost 10%. This is enough to increase the blasttemperature by some 13° C., enough to result in significant coke savingsat the blast furnace.

Computational fluid dynamic (CFD) modelling has been used alongside thedynamic heat balance, to develop a detailed understanding of temporaland spatial variations of temperature, velocity and concentration thatoccur during a complete stove cycle. Some relevant CFD results arepresented in FIGS. 7 to 9. These show that the method according to theinvention can be performed to such similar flame profiles to thoseachieved in conventional operation of the blast furnace stove with airas the oxidant and without recirculation of the flue gas. It cantherefore be inferred that the method according to the invention can beoperated with a stable, visible flame or flames and without generatingpeak flame temperatures likely to damage the stove refractory or checkerwork.

Referring now to FIG. 5, according to a preferred embodiment, enoughcombustion gases are recycled to essentially maintain or increase thegas mass flow per time unit through the refractory material.

According to an alternative preferred embodiment, enough combustiongases are recycled to essentially maintain or increase the thermalenergy throughput through the refractory material. This takes intoconsideration the different heat capacities for various inert componentsin the combustion gases. In this case, it is also preferred that enoughcombustion gases are recycled so that the flame temperature isessentially maintained or decreased.

As is also shown in Table 3, the CO₂ contents of the flue gases ventedfrom the stove 500 are much higher—41% as compared to 23% in theconventional operation mode. The costs per unit weight captured CO₂ forconventional carbon capture techniques is significantly decreased as theCO₂ concentration increases from low levels up to a level of roughly50-60%. Concentrations increased beyond this limit will provide smallergains. As a result, the costs for a carbon capture step for treating thestove flue gases may be reduced significantly per unit weight capturedCO₂ when a high-oxygen oxidant is used in accordance with the presentinvention.

According to a preferred embodiment, an existing, conventional, airburner, which was used to heat the existing stove 500 previously, is inan initial step replaced by a fuel inlet 516 and an inlet for recycledcombustion gases 513, and the fuel is then combusted with the abovedescribed high oxygen oxidant. To this end, it is preferred that theoxidant is submitted by premixing with the recycled combustion gases. Itis alternatively preferred that such premixing is combined with one orseveral lances as described above.

FIG. 6 is an overview illustration of another preferred embodiment ofthe present invention, showing a blast furnace stove 600 with acombustion chamber 601, refractory material 602, a port 606, a conduitfor recycled combustion gases 610, a recycle device 611, a fuel supplyconduit 616, a fuel supply device 617 and a fuel inlet 618.

Oxidant is supplied via an oxidant supply conduit 613 and an oxidantsupply device 614 to an oxidant lance arranged so that the orifice 615of the lance is arranged adjacent to an orifice 612 for supply ofrecycled combustion gases, supplied from the recycle device 611.Preferably, the oxidant lance runs coaxially with the recycledcombustion gas inlet 612. In a way which is similar to the function ofthe coaxial lance orifice 416 as described in connection to FIG. 4, suchan adjacent arrangement, especially when coaxial, will efficientlyentrain the recycled combustion gases into the combustion zone byejector action on the part of the high velocity oxidant, creating morecombustion gas recirculation in the combustion chamber 601. At the sametime, there is no need for a separate propelling device in the recyclingdevice 611, since the recycled combustion gases will be propelled by theejector action at the orifice 615.

The embodiment shown in FIG. 6 is advantageously combined with anadditional oxidant lance, providing additional oxidant at a location inthe combustion zone located at a distance from the orifice 615, therebyachieving a staged combustion in the combustion zone.

As indicated above, it is furthermore preferred that the stove 300, 400,500, 600 is connected to a respective carbon capture step 350, 450, 550,650, which may be conventional per se, separating the carbon dioxidecontents of the combustion gases vented from the stove before thecombustion gases are released into the environment.

When the age of a blast furnace stove approaches its expected usefullife, it is preferred to apply one of the herein described embodiments,or a combination of several of them, to the stove.

This way, the useful life of the stove may be prolonged, operating itwith lower flame temperatures, with maintained production rates in termsof blast air, better fuel economy and lower emissions.

Thus, a method according to the present invention will allow a blastfurnace stove to be operated only on a low grade fuel such as blastfurnace top gas, with no need for higher calorific value fuel enrichmentand no risk for temperature-induced stove damage, while producing fluegases that are better suited for carbon capture. In addition, it allowsthe useful life of a stove to be prolonged.

If sufficient recycling of combustion gases is used, it is also possibleto achieve the same amount and quality of blast air in an existing stovewhich is converted, according to what has been described above, foroperation with a high-oxygen oxidant, and which stove is provided withthe combustion gas recycling arrangement described in connection to FIG.5 or 6. Above, preferred embodiments have been described. However, it isapparent to the skilled person that many modifications may be made tothe described embodiments without departing from the present invention.

For example, any one of the methods for creating recirculation ofcombustion gases as described in connection to FIGS. 4 to 6 mayadvantageously be supplemented with one or several of the variousoxidant lances as described in connection to FIG. 3.

Moreover, the ejector-propelled recirculated combustion gases method asdescribed in connection to FIG. 6 may advantageously be premixed with acertain amount of high-oxygen oxidant in a way similar to the onedescribed in connection to FIG. 5.

Also, the ejector-propelling of pre-mixed or non-pre-mixed recycledcombustion gases as described in connection to FIG. 6 may advantageouslybe combined with ejector-propelling of low-grade fuel as described inconnection to FIG. 4.

Heat may be recovered from combustion gases that are not recycled.Additionally or alternatively, the combustion gas may be subjected tocarbon capture.

The method according to the invention may be applied to Kalugin blastfurnace stoves as an alternative to the stores illustrated in thedrawings.

It will be understood that the embodiments described herein are merelyexemplary, and that one skilled in the art may make variations andmodifications without departing from the spirit and scope of theinvention. All such variations and modifications are intended to beincluded within the scope of the invention as described and claimedherein. Further, all embodiments disclosed are not necessarily in thealternative, as various embodiments of the invention may be combined toprovide the desired result.

1. A method for heating a blast furnace stove, comprising combusting afuel with a lower heating value (LHV) of 9 MJ/Nm³ or less in acombustion region in which there is maintained a stable, visible flame,arranged in a combustion chamber in the stove, and causing thecombustion gases to flow through and thereby heat refractory material inthe stove, wherein the fuel is combusted with an oxidant comprising atleast 85% oxygen, and wherein combustion gases are caused to berecirculated into the combustion region thereby diluting the mixture offuel and oxidant therein sufficiently for the flame not to damage therefactory material.
 2. The method of claim 1, wherein combustion gasesare recirculated from a location inside the combustion chamber, butoutside of the part of the combustion chamber occupied by the combustionregion, and the oxidant is supplied to the combustion zone at highvelocity through a lance, thereby entraining combustion gases into thecombustion zone to achieve dilution of the flame.
 3. The method of claim2, wherein the oxidant is lanced at a velocity of at least 200 m/s. 4.The method of claim 2, wherein the oxidant is lanced at least at sonicvelocity.
 5. The method of claim 2, wherein the lance has an orificeadjacent to a supply inlet for fuel, thereby entraining such fuel intothe combustion region by ejector action.
 6. The method of claim 1,wherein the combustion is staged.
 7. The method of claim 1, wherein as apreliminary step an existing stove is refitted to perform the method,the refitting includes providing an existing burner in one or severalhigh-velocity oxidant lances injecting said oxidant to supplement anexisting burner, the existing burner being placed in communication withthe recirculated combustion gases.
 8. The method of claim 1, whereincombustion gases that have flowed through the refractory material arecaused to be recycled back into the combustion region.
 9. The method ofclaim 8, wherein the recycled combustion gases are premixed with saidoxidant before entering the combustion region.
 10. The method of claim8, wherein enough combustion gases are recycled so that the total oxygenpercentage by volume of the inert part of the atmosphere in thecombustion chamber, excluding counting the non-inert fuel components,does not exceed 12%.
 11. The method of claim 1, wherein an existingstove is as a preliminary step adjusted to perform said method byreplacing an existing air burner with a fuel inlet and an inlet forrecycled combustion gases, and the fuel is then combusted with saidoxidant.
 12. The method of claim 11, wherein enough combustion gases arerecycled to maintain the gas mass flow per time unit through therefractory material at a level which is at least the same as the gasmass flow per time unit which was used when the existing air burner wasoperated without recycling.
 13. The method of claim 11, wherein enoughcombustion gases are recycled to maintain the flame temperature at alevel which is the same or lower, and a thermal energy transfer to therefractory material at a level which is the same or higher, as the flametemperature and the thermal energy throughput per time unit,respectively, which was used when the existing air burner was operatedwithout recycling.
 14. The method of claim 1, wherein the fuel comprisesblast furnace top gas.
 15. The method of claim 14, wherein the blastfurnace top gas is from a blast furnace which is supplied with hot airby the stove.
 16. The method of claim 1, wherein the fuel comprisescalorifically enriched blast furnace top gas.
 17. The method of claim 1,wherein the flame temperature is maintained below 1400° C.
 18. Themethod of claim 17, wherein the flame temperature is maintained below1350° C.